Short answer --
Because complete combustion burns up all the carbon in the oil or coal.

Long answer --
As long as there is a good, strong draft in the firebox from pulling a heavy-enough train, there will be only a slight haze above the stack. Black smoke is UNburned fuel, and it soots up the insides of the tubes and flues. Soot is a very good insulator, and will keep most of heat from the fire in the firebox from being transferred to the water in the boiler. The water in the boiler has to boil hard, and at a far higher temperature than the usual 212 F that most people experience when boiling water for their coffee, or other things.

You will occasionally see some dark gray "smoke" coming from the stack. but that is just the soot that is being scrubbed out of the tubes and flues when the fireman "sands the flues" -- puts scoops of sand into the firebox -- to scrub the soot out of the flues ad tubes.

A properly-fired steam locomotive pulling a heavy train should not make smoke.

All that black smoke that everyone expects was begun and done for many decades because -- until the advent of inexpensive video cameras and now smart phones -- taking moving pictures of a train was very expensive because film was very expensive, and the only way to show that the train was moving when the still picture was taken (when it was too warm for the condensing steam to show) was for the crew to make a plume of black smoke that trailed over the train. IMO, people have been mis-educated by that to think that all steam locomotive have to make black smoke in order to run. Nothing could be further from the truth.

In the steam era, a crew could get into trouble with RR management if they were caught making black smoke needlessly. The SP require a slight haze above the stack, nothing more.

Sorry this is so long, but I wanted to give you and others the reasons why the 4449 did not make black smoke this week. She was being properly run and fired, so -- no black smoke! Just a nice plume of condensing steam -- because the air temp was cold enough for that to show. Beautiful!!

Thank you, Doyle and the 4449 crew, for a job very well done! And thank you, BNSF, for allowing IMAX to do these movie shoots with the 4449, and thank you, IMAX, for including the 4449 in the movie you are making about trains. I can hardly wait to see it! (It will come out some time next year.)